The hard disk drive (HDD) that is one kind of the magnetic recording and reproducing apparatus has the recording density thereof growing at present at an annual rate of 60% or more. The trend of this growth is said to last in the future. Thus, the development of a magnetic recording head and the development of a magnetic recording medium that fit the high recording density are being promoted.
The magnetic recording medium mounted on the magnetic recording and reproducing apparatus currently available in the market is mainly an in-plane magnetic recording medium having the easily magnetizing axis in a magnetic film oriented horizontally relative to the substrate. The term “easily magnetizing axis” as used herein refers to the axis that allows easy orientation of magnetization and, in the case of a Co-based alloy, refers to the c axis in the hcp structure of Co.
In the in-plane magnetic recording medium of this kind, an addition to the recording density results in unduly decreasing the volume of a magnetic layer per recording bit and possibly degrading the read/write property due to the effect of thermal fluctuation. Further, during the augmentation of the recording density, the medium noise tends to increase under the influence of the diamagnetic field that is generated in the boundary region between adjacent recording bits.
In contrast, the so-called perpendicularly magnetic recording medium which has the easily magnetizing axis in the magnetic film oriented mainly perpendicularly, even during the augmentation of the recording medium, suffers only minutely from the influence of the diamagnetic field in the boundary region between adjacent recording bits and, owing to the formation of bright boundary bits, represses the increase of noise. Moreover, it defies the effect of thermal fluctuation because it is capable of repressing the decrease of the volume of recording bits due to the augmentation of the recording density. Such being the case, the perpendicularly magnetic recording has been arresting great attention, and the configurations of a medium fitting the perpendicularly magnetic recording have been proposed in recent years.
In recent years, the feasibility of adopting a single magnetic pole head excelling in the ability to write on the perpendicularly magnetic layer with a view to answering the demand for further augmentation of the recording density of the magnetic recording medium has been being studied. With the object of materializing this single magnetic pole head, the magnetic recording medium which has improved the efficiency of exchange of magnetic flux between the single magnetic pole head and the magnetic recording medium by interposing the so-called lining layer, i.e. a layer formed of a soft magnetic material, between the perpendicularly magnetic layer which is a recording layer and the substrate has been proposed.
When the magnetic recording medium which is merely provided with the lining layer as described above is used, however, it falls short of satisfying the read/write property during the course of reproducing the record, the property of resisting thermal fluctuation and the record resolving power. Thus, the desirability of developing a magnetic recording medium that excels in these properties has been finding recognition.
The reconciliation of the augmentation of the ratio of signal to noise (S/N ratio) during the reproduction, which is particularly important for the read/write property, with the enhancement of the resistance to thermal fluctuation constitutes an essential matter for the sake of the future augmentation of recording density. These two factors have a contradictory relation such that one of them declines unduly when the other is enhanced. The reconciliation of them at a high level poses an important problem.
As one of the problems which encounter the perpendicularly magnetic recording medium, the fact that the use of a magnetic layer of the CoCrPt system which is common to all recording and reproducing magnetic layers results in rendering difficult the acquisition of a proper read/write property because this magnetic layer is deficient in segregation of Cr and insufficient to attain physical separation, fine division and magnetic isolation of magnetic grains may be cited.
In the meanwhile, the utilization of a material containing an oxide in CoCrPt in the magnetic layer of the in-plane magnetic recording medium has been proposed (JP-A 2000-276729, for example).
The magnetic layer of this construction is enabled, by using an oxide instead of relying on segregation of Cr, to attain sufficient separation of grains to a certain extent even in the perpendicularly magnetic medium.
The medium constructed as described above uses a material that decreases the amount of Cr to be added thereto and adds an oxide instead. It, therefore, entails such problems as suffering the coercive force of the magnetic layer to grow excessively and failing to effect thorough recording of data with the head because the smallness of the amount of Cr to be added results in increasing the ratio of Pt in the magnetic grains in the magnetic layer and enlarging the constant of magnetic anisotropy, Ku, of magnetic grains.
It becomes necessary, therefore, to adopt a method for lowering the coercive force of the magnetic layer and effecting thorough recording by resorting to such means as decreasing the thickness of the magnetic layer and increasing the amount of Cr to be added. In the meantime, the fact that the decrease of the thickness of the magnetic layer and the increase of the Cr content result in decreasing the magnetic anisotropy constant Ku of magnetic grains and degrading the nucleation as well entails degradation of the property of thermal fluctuation. Further, the fact that the output during the reproduction of data diminishes results in decreasing the ratio of this output to the system noise inherent in the recording and reproducing system and possibly disrupting the acquisition of a sufficient reproducing property. As a result, the properties to be acquired no longer fit the high-density recording.
In the circumstances, the development of a magnetic recording medium which is endowed with an enhanced property of thermal fluctuation, a sufficient read/write property for high-density recording, a particularly proper data recording property, and a high signal/noise (S/N) ratio during the reproduction has been yearned for.
This invention originated in the appreciation of such true state of affairs as mentioned above and is aimed at providing a magnetic recording medium which possesses an exalted read/write property and an enhanced property of thermal fluctuation and allows information of high density to be recorded and reproduced, a method for the production thereof, and a magnetic recording and reproducing apparatus.